Foot and mouth disease (FMD) is an extremely contagious viral disease of cloven-hoofed ungulates which include domestic animals (cattle, pigs, sheep, goats, and others) and a variety of wild animals. The most prominent disease symptoms in FMDV-infected cattle include vesicular lesions of the epithelium of the mouth, tongue, teats and feet. Although some countries, among them United States, Canada, Mexico, Australia and most of Europe, are considered to be free of FMD, the disease is distributed worldwide and has a great economic impact on the export industry. Indeed, several economically devastating outbreaks have occurred over the past decade on almost every continent.
Currently killed-antigen FMDV vaccines are necessarily produced in expensive biological containment facilities, by growing large volumes (thousands of liters) of virulent FMDV that has been adapted to grow in cells, which can be sometimes difficult. This process has resulted in escape of virulent virus from the manufacturing facility causing costly outbreaks in livestock (see Cottam et al. 2008. PLoS Pathogen 4:1-8). After growth, virus is then inactivated using chemicals and antigen concentrates are prepared, followed by purification steps required to remove contaminant proteins. It is difficult to differentiate infected from vaccinated animals (DIVA) through serological diagnostic tests. There is little to no cross protection across serotypes and subtypes requiring the appropriate matching between vaccine and circulating field strains to achieve protection. Despite these shortcomings of the vaccines, billions of doses are manufactured every year around the world. Their use has been the basis for eradicating FMDV from Europe and for controlling the disease in many parts of the world through mass vaccination campaigns. Creation of genetically engineered viruses containing a backbone and suitable restriction sites partially addresses the shortcomings of inactivated vaccines as restriction sites provide loci for introduction of capsid proteins of different FMD strains. Nevertheless, the cost of antigen is the greatest contributor to the cost of FMD and most other vaccines.
The problem of FMD control is further exacerbated by the phenomenon of virus persistence. Briefly, historically, inactivated FMD vaccines have been unable to prevent persistence or carrier state (defined as virus shedding past 28 days following infection and/or exposure). Shedding animals, while not exhibiting any FMD symptoms, could remain a source of FMD infection to other animals. As such, commonly accepted disease control practices require slaughter of all animals in a vaccinated herd even if they do not have clinical signs of disease.
As such, methods and compositions which lead to vaccines with a lower antigen load without compromising efficiency and/or reducing or eliminating FMD persistence are still desired.